1. Field of the Invention
The present invention relates to an apparatus and method for reducing Peak-to-Average Power Ratio (PAPR) in an Orthogonal Frequency Division Multiplexing (OFDM) system. More particularly, the present invention relates to a linear PAPR reduction apparatus and method for reducing a decrease in throughput and frequency efficiency during PAPR reduction and achieving a low hardware complexity relative to a conventional linear PAPR reduction apparatus.
2. Description of the Related Art
OFDM is a widely used and often discussed technology along with Code Division Multiple Access (CDMA). Because a high-rate serial signal is converted to a plurality of parallel signals and transmitted on a plurality of subcarriers, OFDM is capable of a high data rate, a high frequency efficiency, and robustness against frequency fading channels. However, OFDM has shortcomings in terms of Carrier Frequency Offset (CFO) or PAPR. To keep a channel from impairing orthogonality between subcarriers, OFDM inserts a Cyclic Prefix (CP) in a transmission signal.
In OFDM, a signal is subject to Inverse Fast Fourier Transform (IFFT) prior to transmission on a plurality of subcarriers. A signal resulting from adding a plurality of sine waves to the transmission signal by IFFT has a high PAPR. That is, the PAPR of an OFDM signal is higher than that of a CDMA signal or a signal of any other wired/wireless communication technology, thereby causing non-linear distortion at a High Power Amplifier (HPA) of a transmitter or decreasing the power efficiency of the HPA.
At present, techniques for PAPR reduction in OFDM are under active study. These techniques are categorized into non-linear ones and linear ones. The non-linear PAPR techniques include clipping, clipping & filtering, peak windowing, and peak cancellation, and the linear PAPR techniques include SeLective Mapping (SLM), Partial Transmit Sequence (PTS), and tone reservation.
A non-linear PAPR reduction technique of interest is clipping. In the clipping scheme, when the amplitude of a kth sample sk of an IFFT time signal is larger than a PAPR threshold A, the amplitude of the sample sk is reduced to A in a forced manner, without changing the phase of the sample sk. If the amplitude of the sample sk is less than A, the sample sk is simply output. Despite the advantages of easy implementation and very low hardware complexity, the clipping causes as much non-linear distortion as the difference between the amplitude of the sample sk and A. The non-linear distortion leads to in-band Error Vector Magnitude (EVM) performance degradation and increases adjacent channel out-band emission. As a consequence, spectrum performance is degraded.
                              s          k                =                                                                                              s                  k                                                            ⁢                              exp                ⁡                                  (                                      j                    ⁢                                                                                  ⁢                                          ϕ                      k                                                        )                                                      ⁢                                                  ⇒                                          s                ^                            k                                =                      {                                                                                s                    ,                                                                                                              if                      ⁢                                                                                                s                          k                                                                                                              ≤                    A                                                                                                                                          A                      ⁢                                                                                          ⁢                                              exp                        ⁡                                                  (                                                      j                            ⁢                                                                                                                  ⁢                                                          ϕ                              k                                                                                )                                                                                      ,                                                                                                              if                      ⁢                                                                                                s                          k                                                                                                              >                    A                                                                                                          (        1        )            where sk is the kth sample of the IFFT time signal, A is the PAPR threshold, sk is the clipped signal of sk, and φk is the phase of the kth sample sk.
Although clipping & filtering, peak-windowing, and peal-cancellation have been proposed to overcome the drawbacks of clipping, there still exists non-linear distortion inherent to non-linear PAPR reduction.
A linear PAPR reduction technique of interest is SLM, which will now be described with reference to FIG. 1.
FIG. 1 is a block diagram of a conventional SLM-based PAPR reduction apparatus. Referring to FIG. 1, in SLM, a Parallel-to-Serial (P/S) converter 102 divides an input frequency signal into K paths and multipliers 104 and 106 produce independent signals by multiplying signals in independent paths by different phase shift vectors bk. IFFT processors 108, 110 and 112 IFFT-process the independent signals and a PAPR selector 114 selects an IFFT signal in a path with the lowest PAPR from among the IFFT signals.
As illustrated in FIG. 1, the SLM-based PAPR reduction apparatus requires a total of U IFFT processors 108, 110 and 112, thus increasing hardware complexity. In addition, without knowledge of the path of a signal in a current symbol among the K paths, a receiver cannot demodulate the signal. Hence, side information should be additionally transmitted to the receiver. Thus, SLM decreases the overall throughput of a transmission/reception system.
As described above, in spite of relatively low hardware complexity and simple implementation, the non-linear PAPR reduction techniques suffer from EVM- and out-band emission-incurred performance degradation due to non-linear distortion during PAPR reduction in the transmitter. The linear PAPR reduction techniques, SLM and PTS should transmit information about a phase rotation sequence used for PAPR reduction in every symbol to the receiver, thereby decreasing the throughput of the transmission/reception system. Similarly, tone reservation decreases frequency efficiency because as wide a bandwidth as a reserved tone inserted for PAPR reduction is dissipated. In addition, these linear PAPR reduction techniques have high hardware complexity and are complex to implement.